As for a radiographic image detector so-called Flat Panel Detector (FPD) in which solid-state imaging devices are two-dimensionally disposed, a direct type in which radiation energy is directly converted to electric charge by using a photoconductive material such as a-Se (amorphous selenium) as a radiation detecting element and the electric charge is read as an electric signal in pixel unit by a switch element for signal reading such as TFT (Thin Film Transistor) or the like which are two-dimensionally disposed, an indirect type in which radiation energy is converted to light by scintillator or the like and the light is converted to electric charge by photoelectric conversion elements such as photodiode or the like which are two-dimensionally disposed to be readout as an electric signal by TFT or the like, and the like are well known.
In either of the types, it is known that correction to the photographed image data is needed to be carried out by carrying out a gain correction, an offset correction and the like to the photographed image data which is obtained by detecting radiation which has transmitted an object by a radiographic image detector.
Generally in the correction to the photographed image data, the correction is carried out so as to obtain the final image data Fo(x, y) by subtracting an offset correction value O(x, y) from the photographed image data F(x, y) which is outputted from each radiation detecting element (coordination in the sensor panel unit is (x, y)) of the radiographic image detector and by multiplying the above obtained difference by a gain correcting value G(x, y) as shown in the following formula (1).Fo(x,y)=(F(x,y)−O(x,y))×G(x,y)  (1)
In such way, in the correction of the photographed image data, it is necessary to obtain an offset correction value O(x, y) and a gain correction value G(x, y). Therefore, generally, calibration is carried out periodically to the radiographic image detector to update the gain correction value G(x, y) or the offset correction value O(x, y) or both thereof. As for the offset correction value O(x, y) which has a relatively short variable period (that is, having a greater tendency to vary) comparing to the gain correction value G(x, y), an offset calibration where the radiographic image detector is let stand for a predetermined period of time without having radiation irradiated and the offset correction value O(x, y) is updated by carrying out dark reading which brings out electric charge accumulated in the radiation detecting element is carried out often times in order to know the varying of characteristic over time of the offset correction value O(x, y).
Moreover, in order to remove unevenness in a photographed image by canceling an effect of temperature characteristic and characteristic variation of each of the elements such as a radiation detecting element, TFT (Thin Film Transistor) which is a switch element for signal reading or the like and an effect of residual potential due to previous radiation irradiation (photographing) and the like, there is a case where the offset correction value O(x, y) of the radiographic image photographing is to be calculated by detecting an output value (hereinafter, called dark read value D(x, y)) from each radiation detecting element in a state where radiation is not irradiated just before or just after the photographing for each radiographic image photographing.
This process is for obtaining the offset correction value O(x, y) under a temperature condition as much as same as the temperature condition of the radiation detecting element at the time when the photographed image data F(x, y) was obtained in the radiographic image photographing.
Similarly to when obtaining photographed image data F(x, y), various types of electrical noises such as dark current noise of photodiode and the like, TFT transient noise, TFT thermal noise, TFT leak noise, thermal noise which occurs due to a parasitic capacitance of a data line that reads electric charge from TFT, amplifier noise of inside of a readout circuit, quantization noise which occurs due to A/D conversion and the like have influence when obtaining the dark read value D(x, y). Therefore, even when the dark read value D(x, y) is read under the same temperature condition, fluctuation (variation) occurs in signal values due to the electrical noises in the dark read value D(x, y). Thus, even when the dark read value D(x, y) is read just before or just after the radiographic image photographing, the dark read value D(x, y) which is read is not necessarily the true value of the offset correction value O(x, y) under the photographing condition such as temperature condition and the like.
Therefore, in many cases, dark reading is to be carried out for a plurality of times and the average value of each of the dark read values D(x, y) is to be calculated to use the average value as the offset correction value O(x, y). (For example, see Patent Documents 1 to 3).
This is based on a consideration that when the average value of the dark read values D(x, y) which are readout in a plurality of times of dark readings is calculated, fluctuation of each of the dark read values D(x, y) is to be alleviated or is to be cancelled out. Therefore, the average value is practically the true value of offset correction value O(x, y) under the photographing condition or at least the average value is a value close to the true value of offset correction value O(x, y). Further, when the photographed image data F(x, y) is corrected by using the offset correction value O(x, y) which is the average value, S/N ratio of the final image data Fo(x, y) after the correction can be a favorable ratio.
On the other hand, conventionally, the image processing such as correction processes and the like including the above offset correction are carried out in processing devices such as an image processing processor, a console and the like which are different from a photographing device such as the radiographic image detector and the like in many cases (for example, see Patent Document 4). Further, in recent years, there is developed a portable radiographic image detector in which a battery is built-in and which carries out sending and receiving of photographed image data F(x, y) and the like with an external processing device or the like by a wireless method without using a cable (for example, see Patent Document 5).    Patent Document 1: U.S. Pat. No. 5,452,338, specification    Patent Document 2: U.S. Pat. No. 6,222,901, specification    Patent Document 3: U.S. Pat. No. 7,041,955, specification    Patent Document 4: JP H11-113889, publication    Patent Document 5: JP H7-140255, publication